In recent years, as semiconductor devices have been miniaturized, an ALD method has received attention in a film forming process due to its low film forming temperature and good step coverage.
When the ALD method is used to form a film, a plurality of raw material gases is alternately supplied into a chamber which accommodates a semiconductor wafer (hereinafter referred to as a “wafer”) as a workpiece. For this gas supply, in a case where a liquid raw material or a solid raw material whose vapor pressure is low is used as a film forming raw material, a method of bubbling the raw material accommodated in a container with a carrier gas for supply into the chamber has been known. In addition, a method of vaporizing a liquid raw material with a vaporizer for supply into the chamber by a carrier gas has been known. In this method, while vaporizing the liquid raw material in a container and regulating a supply pressure of the raw material gas to a desired pressure by means of an automatic pressure regulator, the raw material gas is supplied into the chamber at a preset flow rate, so that a desired flow rate of the raw material gas can be supplied without using the carrier gas.
However, in the case of the ALD method, it is necessary to switch on/off a raw material gas in a short time and supply the raw material gas at a high flow rate. However, the supply of liquid raw material or solid raw material having a low vapor pressure according to the above-described method has the following problems.
That is, in the method of bubbling the raw material with the carrier gas to supply the raw material gas, conventionally, the raw material gas sent from a container for bubbling is switched on/off by an opening/closing valve, which is hard to supply the raw material gas at a high flow rate while switching on/off for a short time. In addition, in a case where the supply pressure of raw material gas is controlled by an automatic pressure controller without using a carrier gas, it is also hard to supply the raw material gas at a high flow rate while switching on/off for a short time. In addition, in a case of the method of supplying a raw material gas, which is obtained by vaporizing a raw material by means of a vaporizer, with a carrier gas, since the raw material cannot be vaporized if a secondary pressure at the rear side of the vaporizer is high, an opening/closing valve installed in a gas supply pipe guiding the raw material gas into a chamber and an opening/closing valve installed in a bypass pipe bypassed from the vaporizer to an exhaust line are alternately switched on/off to switch on/off the raw material gas. In such case, since pressure variation in the interior of the vaporizer is accompanied, a pressure cannot be raised up to the maximum vaporization partial pressure, which naturally causes a limit in the supply amount of gas. In addition, during a period of time that the raw material is not supplied into the chamber, the raw material gas is discarded, wasting a raw material.